DC to DC (DC-DC) converters operate to convert an input DC voltage to an output DC voltage with isolation between the input and output. Such converters normally utilize high frequency switching technique. There are many different types of converters each having specific advantages and disadvantages. The present invention is directed to a self-excited DC-DC converter and a power supply device using such a DC-DC converter.
FIG. 5 shows an example of a self-excited DC-DC converter in the conventional technology. In this example, the self-excited DC-to-DC converter has a series circuit connected between input terminals t1 and t2 and formed of a PNP transistor Q1, a primary winding L1, and a capacitor C2, a voltage drop chopper circuit formed of a diode D1 whose cathode is connected to the connection point of the transistor Q1 and the primary winding L1 while its anode is connected to the connection point (ground) of the capacitor C2 and the input terminal t2. A feedback winding L2 that is magnetically coupled to the primary winding L1 is connected across a base and an emitter of the transistor Q1 through a capacitor C1.
Further in FIG. 5, a series circuit comprising resistors R5 and R6 connected between both terminals of the capacitor C2. The base of an NPN transistor Q7 connects with the connection point of resistors R5 and R6. The collector of the transistor Q7 connects to the base of a transistor Q6 as well as to the emitter of the transistor Q1 via a resistor R10. The collector of the NPN transistor Q6 connects with the base of the transistor Q1 through the resistor R9, and the emitter of the transistor Q6 connects with the ground.
The operation of this self-excited DC-DC converter is described below. FIGS. 6(a) to 6(d) show the waveforms involved in this circuit. When the DC input voltage source E is supplied to the input terminals t1 and t2, a base current flows to the transistor Q6 through the resistor R10, thereby turning on the transistor Q6. As the transistor Q6 turns on, the base current I4, which is a difference between I2 in FIG. 6(c) and I5, flows to the transistor Q1 through the transistor R9, and the transistor Q1 turns on. As the transistor Q1 turns on, the collector current I1 shown in FIG. 6(b) flows in the transistor Q1, thereby generating a potential difference across the primary winding L1.
Accordingly, a potential difference is also generated across the feedback winding L2 that is magnetically coupled to the primary winding L1. The transistor Q1 is biased due to the potential difference across the feedback winding L2, which immediately turns on the transistor Q1. At this time, the capacitor C2 charges electricity via the path formed from the input voltage source E, transistor Q1, primary winding L1, capacitor C2, to input voltage source E. Thus, energy is provided to the capacitor C2 and a load circuit (not shown) connected across output terminals t3 and t4.
The collector current I1 flowing through the transistor Q1 increases by the parameter determined by the inductance of the primary winding L1. Since the base current I4 of the transistor Q1 is determined by the resistor R9 and the transistor Q6, when the current I1 becomes larger than I4.times.h.sub.FE (where h.sub.FE is a current gain in the transistor Q1), the base current I4 fails to keep the saturation of the transistor Q1, and the transistor Q1 goes to the non-saturation region. Thus, the voltage V1 (collector-emitter voltage of the transistor Q1) shown in FIG. 6(a) increases.
As the voltage V1 increases, the voltage across the primary winding L1 decreases, and the voltage across the feedback winding L2 which is magnetically coupled to the primary inductor L1 also decreases. Accordingly, the base current I4 of the transistor Q1 decreases, resulting in the further increase in the voltage V1 across the collector-emitter of the transistor Q1, which turns off the transistor Q1 rapidly. At this time, the energy stored in the primary winding L1 when the transistor Q1 is on transfers to the capacitor C2 and the load circuit through the path formed by the primary winding L1, capacitor C2, diode D1, and primary winding L1. When the energy stored in the primary winding L1 is completely discharged, the base current flows again to the base of the transistor Q1. The oscillation in this circuit continues by repeating the process described above. The current I3 in the diode D1 is shown in FIG. 6(d).
In the example of FIG. 5, the DC-DC converter circuit controls the base current of the transistor Q6 by a circuit arrangement comprising resistors R5, R6 and a transistor Q7 based on an output voltage, and thus controls the base current I4 of the transistor Q1. For example, when the output voltage increases due to a decrease in the load, the voltage at the connection point of the resistors R5 and R6 increases. Hence, the current flowing through the transistor Q7 increases, which decreases the base voltage of the transistor Q6 as well as the base current of the transistor Q6. Since the collector current of the transistor Q6 decreases, the base current I4 of the transistor Q1 also decreases, which quickens (advances) the timing of the transistor Q1 to go to the off state. Therefore, the peak value of the current I1 decreases, thereby limiting the increase of the output voltage.
On the other hand, when the output voltage decreases due to an increase in the load, the voltage at the connection point of the resistors R5 and R6 decreases. Hence, the current flowing through the transistor Q7 decreases, the base voltage of the transistor Q6 increases, and the base current of the transistor Q6 increases. Accordingly, the base current I4 of the transistor Q1 increases, which delays the timing of the transistor Q1 to go to the off state. Consequently, the peak value of the current I1 increases, thereby limiting the decrease of the output voltage.
As described in the foregoing, in this circuit, the circuitry comprising the resistors R5, R6 and transistor Q7 controls the base current of the transistor Q6 based on the output voltage, and controls the base current I4 of the transistor Q1, thereby maintaining the constant output voltage to the load circuit. The more detailed description of this conventional DC-DC converter circuit is given in the Japanese Patent Laid-Open Publication No.5-2585.
The conventional DC-DC converter described in the foregoing enables to maintain the constant output voltage with a simple circuit configuration. However, when the input voltage source E is produced from the wide voltage range of commercial power sources such as AC100V-AC240V by rectifying and smoothing the same, a high voltage may be applied to the resistor R9 and the transistor Q6 which control the base current I4 of the transistor Q1.
Because of the high voltage, a large base current ranging from several mA (milliampere) to several ten mA flows in the transistor Q6, resulting in the increase in the heat dissipation and power loss by the transistor Q6. In addition, components of high breakdown voltage must be used, which increases the overall size of the converter circuit. Moreover, because the current flowing through the transistor Q7 changes in response to the voltage level of the input voltage source E, constant voltage feedback by the transistor Q7 is unavailable.